1. Field of the Invention
This invention relates to an improved electrical machine, and more particularly to such a machine having a rotor, a stator, and a permanent magnet located on the rotor.
2. Description of the Prior Art
Electrical machines are well known from the prior art. For instance, permanent magnetically excited direct current motors are known that can be commutated electronically or mechanically. A distinction can be made between two kinds of embodiment, namely one in which the permanent magnets are located on the stator, and another in which the permanent magnets are mounted on the rotor. The commutated magnetic field between the permanent magnets and the coils disposed on the other component generate the torque on the rotor shaft.
For securing the permanent ring magnet to a rotor body of the rotor shaft, it is known for instance from European Patent Disclosure EP 0 872 945 A1 to use an adhesive bond. Because of the different coefficients of expansion in response to temperature of the various materials comprising the magnet, adhesive, and rotor body, and because of production variations and because of the requisite spacing, when an adhesive is used, between the magnet and the rotor shaft/body, major stresses in the various materials (magnet, adhesive, shaft) occur at the bonded faces, on the one hand. On the other, however, during operation of the electrical machine major temperature differences can occur, which because of the different coefficients of expansion of the materials causes cracks, to the extent of breakage of material, so that it is no longer possible to transmit torque from the magnet to the rotor shaft. Furthermore, the mechanical properties of the adhesive become poorer as the temperature rises. At this time, the adhesive is the most-critical link in the entire chain.